1. Field of the Invention
This invention generally pertains to horn antennas and more specifically to a horn antenna having an in-line balun connected to a cornu spiral flare.
2. Description of the Related Art
Good ultrawideband (UWB) antenna performance has been defined as relatively constant frequency-domain parameters such as impedance, pattern, gain, and polarization over at least 25 percent fractional bandwidth. The fractional bandwidth being defined by the upper frequency subtracted by the lower frequency and divided by the center frequency. From a time-domain perspective, good UWB performance can be defined as efficiently radiating or receiving a short pulse of electromagnetic energy with a small amount of signal distortion. Wideband efficiency is defined as the ratio of the total energy radiated to the total energy incident on the antenna, and is limited by energy reflected from the antenna back down the feed transmission line. See, Lamesdorf et al., Baseband-Pulse-Antenna Techniques, IEEE Ant. & Prop. Mag., Vol. 36, pp. 20-30, Jul. 1994. The amount of reflected voltage at a particular frequency is measured by a quantity called the voltage standing wave ratio (VSWR). A VSWR of 1.0 means no reflected voltage, whereas any value greater than 1.0 means energy that is reflected and therefore not radiated. Thus, a lower VSWR is almost always desirable. Ohmic, or heating, losses also hinder antenna efficiency, but in cases of antennas constructed only of highly conducting material, a high VSWR is normally a much more serious problem. In general, a good time-domain UWB antenna will also be a good frequency-domain antenna, but not necessarily vice-versa. For example, spiral antennas are noted for good UWB frequency-domain behavior and yet are poor UWB pulse antennas. Thus, it is necessary to evaluate time-domain UWB performance to demonstrate the antennas use in pulse applications. However, researchers have only recently investigated UWB antenna pulse behavior.
With the advent of large memory computers with fast processors, numerical modeling with time-domain Maxwell's equations algorithms such as the Finite-Difference Time-Domain (FDTD) method has led to the testing of antenna designs on computer platforms. See, Yee, Numerical Solution of Initial Boundary Value Problems Involving Maxwell's Equations in Isotropic Media, IEEE Trans. Antennas Propagat., Vol. 14, pp. 302-307, May 1966. Antenna designs based on FDTD calculations have been verified time and time again on antenna ranges.
Robertson and Morgan outlined the frequency-domain characteristics necessary for the transmission of a pulsed signal with no distortion. See, Robertson et al., Ultra-wide-Band Impulse Antenna Study and Prototype Design, Tech. Rpt. No. NPSEC-93-010, U.S. Navy Postgraduate School, Mar. 1993. These frequency independent characteristics include a complex conjugate match between the source impedance and the input impedance of the antenna, a constant gain over a preferred angular sector, and an effective height with a linear phase response, where the effective height is defined as the ratio of the open circuit receive voltage to the incident electric field. To receive a voltage that is a scaled replica of the incident field time-variation, an antenna should have a gain proportional to the square of the frequency as well as an effective height with a linear phase response. Because of the differing gain requirements on transmit and receive, it is not possible for a single antenna operating over a given pulse bandwidth to both radiate a far-zone field that is a scaled replica of the input voltage signal and to receive a voltage that is a scaled replica of the incident field. It is desirable, however, to achieve predictable antenna time-domain behavior.
Kraus points out that it is possible to deduce the qualitative behavior of an antenna from its appearance. See, Kraus, Antennas, McGraw-Hill, New York, N.Y., 2nd Ed., 1988. In particular, flared antennas which have a twin-conductor transmission line separation much less than a wavelength at the highest frequency, an aperture size greater than a wavelength at the lowest frequency, a constant characteristic impedance up to the aperture, and no discontinuities will tend to have wideband characteristics.
Transverse electromagnetic (TEM) horns are noted for high power wideband pulse performance. Most TEM horn antennas have a gain nearly proportional to the square of the frequency and thus the antenna will tend to transmit a far-zone field on the boresight that is nearly a scaled temporal derivative of the input voltage and receive a voltage that is nearly the scaled replica of the incident boresight field. Boresight is defined as the direction about which the antenna pattern is most symmetric. Flared horns, which contain TEM horns as a subclass, will also tend to transmit a field that is nearly a scaled temporal derivative of the input voltage signal on boresight and receive a voltage that is nearly a scaled replica of the boresight incident field. When implemented properly, the far zone fields have a broad angular distribution of significant radiated energy and high far-field fidelity, where fidelity in this case is defined as the degree to which the radiated fields are a scaled temporal derivative of the input voltage or the degree to which the received voltage is a scaled replica of the incident field.
FIG. 1 shows the salient features of a standard TEM linear flared-horn antenna. In this basic configuration, the antenna 10 is transverse-fed by a coaxial transmission line 12 TEM mode into the parallel-plate feed region 32. The plates 16 and 18 are transitioned at the feed to flare discontinuity 24 to the linear horn 34 flared out along the boresight direction 22 of radiation. The primary factors that affect the flared horn radiation of an UWB signal include the waveguide modes present in the feed region 32, the spatial point-to-point characteristic impedance of the transmission line 12, 32, and 34, the discontinuities 15 and 17 present at the aperture plane 28 and feed termination discontinuity 14 of the antenna, the flare taper shape and length 34, and the aperture size 28.
Several waveguide modes can exist in the parallel-plate feed region 32 of the antenna depending on the geometry and source fields, although the primary mode is the TEM mode. For the TEM mode, all frequency components will propagate down the waveguide at the same velocity, so they will tend to arrive at the flare region 34 in time synchronization. Higher order TE.sub.On and TM.sub.On modes travel down the waveguide at velocities that vary with frequency. As a consequence, if parallel plates 16 and 18 support TE.sub.On and TM.sub.On modes, the propagating signal will tend to distort. A parallel plate waveguide will cut off higher order mode propagation above the wavelengths given by the formula ##EQU1## where .lambda..sub.cutoff is the wavelength corresponding to the cutoff frequency, S is the plate separation 38 in the same units as .lambda..sub.cutoff, and n is the mode number. See, Foster Ed., Introduction to Ultra-Wideband Radar Systems, Ch. 5, pp. 145-216, CRC Press, Inc., Ann Arbor, Mich., 1995. Thus to propagate only a TEM wave, the parallel plate waveguide should have a plate separation 38 no larger than one-half wavelength at the highest frequency component of the input signal. The width 36 of the parallel plates 16 and 18 sets the characteristic impedance and should be chosen for maximum or constant wideband energy transfer from the coaxial line 12.
The flare taper known as the cornu spiral has never been applied to flared horn antenna design. It has long been recognized by antenna designers that sharp geometrical discontinuities cause reflections that raise VSWR and thus constrain the amount of radiated energy. A cornu spiral is formed by parametrically plotting the scaled cosine Fresnel integral against the scaled sine Fresnel integral, and it has the unique property that the curvature of the flare increases linearly in proportion to the arc length of the flare. See, Jahnke et al., Tables of Higher Functions, McGraw-Hill N.Y., 6th ed., pp. 28-30, 1960. The cornu flare taper satisfies an important criteria in a wideband design, which is the notion of a smoothly varying geometrical change to limit diffraction and reflection. In addition the cornu flare arms spiral behind the aperture plane, so a sharp geometrical discontinuity at the aperture is avoided. By contrast, the TEM horn terminates abruptly at the aperture plane, which causes diffraction and reflection of waves, and raises VSWR.